The invention relates to a rotary optical coupler between a first array of optical conductors and a second identical array of optical conductors, which optical coupler comprises an optical body for a mirror-symmetric transfer of radiation between the two arrays of optical conductors, said optical body being further capable of rotating about the rotation axis of the optical coupler at the same angular velocity but in opposite direction with respect to the two arrays of optical conductors. An optical conductor is here understood to be in particular an optical fibre guide.
The rotary optical coupler is utilised for the transfer of digital data between mutually rotatable units, and is known from the U.S. Pat. No. 4,027,945. In this patent the optical body consists of a prism, preferably a Dove prism or a Pechan prism. The application of a Dove prism has however the disadvantage that, due to the reflection of the light at each of the two end faces, the path of the individual light rays is frequency-dependent. Consequently, in view of the fixed arrays of the optical conductors, the light intensity of the radiation transferred is frequency-dependent. The application of a Pechan prism on the other hand has the disadvantage that, due to a five-fold reflection of the incident light and the associated long path length, the intensity of the light transferred is reduced. Also, because of the large ratio between the optical path length within the prism and the width of the prism, the optical conductors have to be aligned very accurately, or the permitted number of optical conductors will be limited. Further, in both cases it is desirable to dispose the end members of the individual optical conductors at some distance from each other to avoid crosstalk between the various conductors. Other types of prisms have similar disadvantages.